The wireless communications network has grown exponentially over the years. A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simplified network architecture. LTE systems, also known as the 4G system, also provide seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNodeBs or eNBs) communicating with a plurality of mobile stations, referred to as user equipments (UEs). The 3rd generation partner project (3GPP) network normally includes a hybrid of 2G/3G/4G systems. With the optimization of the network design, many improvements have developed over the evolution of various standards.
As set forth in the 3GPP, IP Multimedia Subsystem (IMS) is a core network that provides IP multimedia services to user equipments (UEs) over an Internet Protocol (IP) network. Historically, mobile phones have provided voice call services over a circuit-switched (CS) network, rather than strictly over an IP packet-switched (PS) network. Alternative methods of delivering voice or other multimedia services over IP have become available on smartphones (e.g. VoIP or Skype), but they have not become standardized across the industry. IMS is an architectural framework to provide such standardization. IMS is able to communicate with UEs through different types of access network, such as a wireless local area network (WLAN), an Ethernet network, a packet data network (PDN), or another type of access network. IMS is a new way to dial PS call on LTE (Voice over IP or Voice over LTE) instead of fallback to 2G/3G legacy CS call.
The Next Generation Mobile Network (NGMN) board, has decided to focus the future NGMN activities on defining the end-to-end requirements for 5G. Voice service will be an important feature for the next generation system, e.g., NG system (NGS) or 5G system (5GS). It is proposed that the NG/5G systems shall support IMS PS voice service, IMS PS voice service continuity with the 4G evolved packet system (EPS), and IMS PS voice service fallback to EPS. However, when a voice call is triggered, a UE may be in conditions where voice service is not supported. In this case, the UE shall move to a proper access or core system to continue the voice call setup.
Solutions for IMS call setup in NGS/5GS when the serving cell in NGS/5GS is unable to support multimedia calls are sought.